Diode pumped alkali lasers (DPAL) are a rather recent development in laser technology. The first DPALs were suggested and demonstrated about a decade ago. Over this time, the average power produced from DPALs has increased from milliwatts to more than 200 W. Initial experiments were all performed in static systems. As the system power increased, DPALs were considered for potential use in DPAL-based weapon systems. The heat generated in the gain medium became a significant problem as the power capability of DPALs increased.
One solution for removing heat in high power DPALs is to flow the gas gain medium. However, a gas flow system in applications involving alkali metals is complicated. One problem is material compatibility: alkali metals are highly reactive and the appropriate materials must be used for all parts of the system. This eliminates using many types of lubricants and elastomeric materials. In addition to reactivity, alkalis can alloy with certain metals, such as gold (a common component of brazing compounds), and then transport the metal throughout the system. This can lead to problems when the alloys become contaminants that are deposited on the optical windows of the laser. A DPAL system uses optically transparent windows, which undergo a high level of optical irradiation. The continued deposition of contaminants on the windows may eventually result in optical damage and failure.
In addition to the compatibility issues, gas flow systems involving alkali vapors have required very careful thermal control throughout the system. The alkali pressure is given by the vapor pressure of the liquid at the operating temperature. The alkali metal will seek out the coldest region in the system and condense there. The temperature at this spot will determine the overall system pressure. An unexpected cold spot can lower the alkali pressure by many orders of magnitude since the temperature-vapor pressure relation is highly non-linear.
Known closed alkali flow systems run the entire system at high temperature. If all of the components are at an elevated temperature, there is no concern over alkali condensing within the flow system. Such systems contain an alkali reservoir maintained at a temperature that is lower than the rest of the system. This temperature determines the alkali density within the overall flow system. However, operating at high temperature causes additional problems. In general, the helium pump is a high speed piece of rotating machinery. High temperature and alkali metals may make the rotor and bearing engineering difficult. The complex nature of the pump may make it difficult to ensure that a system cold spot does not reside inside of the pump. Highly engineered bearings are typically needed in order to provide proper support and stability to the rotor. These bearings have tight tolerances and limited temperature ranges under which they will operate. High temperature operation may lead to excessive wear and premature failure.
Another problem with DPAL systems is the degradation of the optical windows due to reaction with the alkali metals. Even if the window is made of robust materials, such as sapphire, operation under intense laser irradiation can result in damaged windows when exposed to alkali vapors. While the exact causes are still under investigation, chemisorption of the alkali on to the surface of the window appears to play an important role. Studies have demonstrated that internal gas curtains, consisting of a thin layer of pure helium between the optical window and alkali vapor, mitigate this problem. Gas flow systems that use gas curtains typically employ a blowdown design, which is an open system in which the gas that flows in the laser head is exhausted. Pure helium is introduced from outside of the flow system, which would be difficult to do in a closed loop since it would require removal of an amount of gas equal to the amount of helium introduced into the system. On the other hand, closed loop system are especially desirable for compact airborne systems since they do not require a large supply of gas and the associated high pressure cylinders, which are large and heavy.
In view of the foregoing, there is an ongoing need for systems and methods for flowing alkali vapors in a closed loop. in a way that would permit the use of an inert gas to protect optical windows using gas curtains.